The huge capex of designing and taping out a chip generally means that you wanna make and sell as many of said chip as possible.
Google wants to rent as many TPUs as possible.
Only if you think the external market is relevant in comparison to Google itself.
Every TPU they keep for Google Cloud reduces the need to buy GPUs from Nvidia. Or really, in this market, means they have additional ability to support AI compute beyond what they can buy from Nvidia.